Despite numerous attempts to explain the decrease in resting energy expenditure (REE) with age, and substantial variations in REE that occur between races, the mechanisms involved remain elusive. The loss of fat fee mass (FFM) cannot fully explain the decrease in RE in elderly. Since the FFM compartment consists of numerous tissues and organs, each with a different oxidative metabolic capacity, the possibility exists that changes in the proportion of these tissues with age may explain the lower REE in elderly. Moreover, REE is lower in African-American (AA) compared to Caucasian (C) metabolism at the organ/tissue level using non-invasive techniques are limited. Magnetic resonance imaging (MRI) allows for the complete reconstruction of organ and tissue volumes in vivo in combination with gated cardiac MRI. Project 4 aims are to test whether: 1) a portion of age-related changes in REE are explainable by changes in the relative fractions of high (brain, heart, liver, kidney) and low (skeletal, muscle, adipose tissue) metabolic activity tissues with respect to oxidative metabolism, 2) the cell mass fraction of skeletal muscle is reduced in elderly compared to young women and using a regional K+ counter and MRI, 3) a portion of age-related changes in REE can be explained by reductions in the metabolic activity of brain and myocardial tissues using positron emission tomography (PET) for the in vivo quantification of labeled 150 uptake, 4) there is a non-significant contribution of race to REE after appropriately measuring specific metabolically active compartments (liver, brain, kidneys, heart, skeletal muscle, adipose tissue) in AA and C adults. One hundred and sixty (20- 80 years) AA and C, healthy men and women will be measured, equally distributed to produce a 2 X 2 factorial design. A subset (n=70) of subjects will undergo additional dynamic PET studies combined with 15/O steady-state inhalation techniques to quantify brain and myocardial oxygen consumption. The proposed studies promise to anser important body composition related questions regarding the genesis of age and race differences in REE and other significant biological information.